The turbine of a jet engine provides the power necessary to drive the compressor and accessories, and, in engines which do not make use solely of a jet for propulsion, the turbine provides the power to drive the shaft of a propeller or rotor. Energy produced from the continuous flow of hot gases released by the combustion system of the engine is extracted by the turbine which expands the gases to lower pressure and temperature. In order to produce the driving torque required in the jet engine, turbines normally consist of several stages. Each stage of the turbine employs one row of stationary nozzle guide vanes fixedly mounted to the turbine case, and a rotor which includes a row of rotor blades circumferentially mounted to a rotating turbine disk. The turbine disk is either formed integrally with or is bolted to a turbine shaft.
The blades in the rotor of the turbine each have a blade root adapted to mount to the turbine disk and an airfoil extending radially outwardly from the root which terminates at a blade tip. In many jet engines, the low pressure stage rotor is formed with a shroud which comprises separate segments or tip shrouds mounted at the tip of each rotor blade.
In view of the high rotational speeds of the turbine rotor blades and the mass of the materials which form the blades, proper balancing of the rotors of the turbine is extremely important. Any unbalance can seriously affect the rotating assembly bearings and engine operation.
Conventionally, balancing of the rotor in the low pressure stage of the turbine has been performed in one or two different ways. In one method, weights are bolted to the aft flange of the last stage low pressure turbine disk at one or more locations about its circumference. Each weight produces a moment about the center of rotation of the turbine disk which is the product of the mass of the weight and its distance from the center of rotation. The number, position and mass of the weights are determined by conventional balance testing of the turbine to achieve the desired balancing of the rotor.
One problem with this method is that a relatively large amount of weight is often needed on the aft flange of the turbine disk in order to balance the rotor. This is because the radius or moment arm between the center of rotation of the turbine disk and its aft flange is small. In order to increase the moment produced by the weight over such a short moment arm, the magnitude of the weight must be substantial.
Another problem with mounting balance weights to the aft flange of the turbine disk is that the relatively small radius between the center of rotation of the turbine disk and its aft flange makes it difficult to accurately mount the weights on the aft flange at the desired angular position relative to the center of rotation of the disk. This is particularly true for relatively small angular adjustments, e.g., 1.degree. or 2.degree., wherein the weights can be moved only a very small distance along the circumference of the aft flange to produce the desired angular adjustment relative to the center of rotation of the turbine disk.
Additionally, last stage low pressure turbine disks may not be originally designed with an aft flange and one must be machined into the disk to employ this method of balancing. This machining operation greatly increases the cost of manufacture of the turbine disk and may result in a waste of money where the rotor does not need balancing because the aft flange serves no other function except for use in balancing the rotor.
A second method of trim balancing the last stage low pressure turbine rotor has been to wrap a thin wire about the airfoil of the rotor blade. Typically, the wire is initially wrapped around the root of the blade where it is connected to the turbine disk, but during operation of the engine the wire tends to slide radially outwardly along the airfoil due to the centrifugal force produced by rotation of the blade. This forces the wire against the tip shroud at the radially outermost end of the airfoil of the rotor blade, or somewhere along the airfoil of the blade.
One problem with the use of such wires to balance the rotor is that they tend to fatigue and break away from the rotor blades after a period of engine operation. Movement of the wires along the airfoil of the blades at different operational speeds weakens the wires and often causes them to detach from the blades. Additionally, even if the wires remain attached to the blades, some of the wires can become hung up at various locations which can result in unbalance of the rotor.
A problem common to both of the methods of rotor balancing described above is that the installation of either weights or wires require disassembly of at least a portion of the aft end of the engine. This adds substantially to the time and cost involved in balancing the turbine rotor blades.